Coloured feathers readied first birds for take off

We could be able to work out the colours of extinct animals from the shapes of their pigment factories

(Image: Clarke et al.)

Before the first birds could take flight, they needed a lift from an unexpected source&colon; colourful feathers.

As well as giving birds the most colourful plumage on earth, it seems pigment factories in their feathers primed their feathery dinosaur ancestors for flight by creating feathers of many different shapes. The same pigment factories may also have turbocharged the proto-birds’ metabolism, helping them into the air.

This insight comes from a study of cellular pigment factories called melanosomes from the feathers, hairs and skin of 181 living birds, mammals and reptiles, plus 13 fossils of ancient lizards, turtles, dinosaurs and pterosaurs.

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Julia Clarke of the University of Texas at Austin and her colleagues found that melanosomes suddenly became much more diverse in the lineage leading to birds. This happened at the same time that feather-like appendages appeared in maniraptoran dinosaurs, the forerunners of birds. These “pinnate” feathers resembled the familiar branched structure of modern bird feathers, and contained vastly more diverse melanosomes, in terms of their length, width and shape than the samples from ancient lizards and dinosaurs.

“We found a much greater array of sizes and shapes, ranging from small and round to long and rod-like,” says Clarke.

Colour and speed

The melanosomes from the maniraptoran dinosaurs were of a form that could be linked to colour in modern birds and mammals. That suggests we could reconstruct the colours of extinct animals using the shapes of their melanosomes, says Clarke.

By contrast, the melanosomes from fossil lizards, turtles and crocodiles were all quite similar, and did not correlate with colour in their living counterparts.

But it’s not just about feathers. The melanosome diversity explosion may also have led to higher metabolic rates in the forerunners of birds. Such rapid metabolisms are essential for powered flight.

“Many of the genes involved in the melanin colour system are also involved in other core processes such as food intake, stress responses, reproductive behaviour and more,” says Clarke. That means the change in pigment could be linked to larger changes in the animals’ energetics and physiology.

This is corroborated by the modern animals, where melanosome diversity is linked to metabolism. “Only in living warm-blooded terrestrial vertebrates which independently evolved higher metabolic rates did we find the melanosome diversity we also see in feathered dinosaurs,” says Clarke.

“The authors have sampled extremely widely among living species, so the case couldn’t be stronger,” says Mike Benton of the University of Bristol, UK. “That lends weight to their claim that the link between melanosome and external colour arose in the forerunners of birds.”